This application claims priority under 35 U.S.C. xc2xa7119 on German patent application number DE 10145184.9 filed Sep. 13, 2001, the entire contents of which are hereby incorporated herein by reference.
The invention generally relates to a method of laser drilling of electrical circuit substrates. More preferably, it relates to a method wherein a laser beam is directed onto an intended portion of the substrate, preferably through a perforated mask close to the surface, in order to drill a hole with a predetermined hole diameter.
The use of perforated masks when drilling holes, in particular also blind holes, in printed circuit boards and comparable substrates which serve as leadframes has long been known (U.S. Pat. No. 4,644,130A). The generally metallic perforated mask has apertures, which define by their configuration both the position and size of the holes to be drilled (known as the conformal mask). The metal layer which forms the mask, known as the proximity mask, is in this case placed on the surface of the substrate, directly in contact with its surface or at a small distance from it.
In U.S. Pat. No. 5,585,019A, a description is given of a laser machining method using a device in which it is ensured by the design of the optics for the laser beam that the laser beam always impinges on the substrate at the same angle, without the latter itself having to be moved. When drilling printed circuit boards with a perforated mask, the laser, usually a CO2 laser or a laser with comparable properties, is always pulsed on the same point via the perforated mask (known as punching). To achieve material removal which is as uniform as possible over the entire cross section of the hole during this operation, it is aimed to achieve for the laser beam an energy distribution which is as uniform as possible over the entire diameter of the hole, while lowest possible energy losses outside the edge of the hole are desired. The energy distribution is accordingly intended to have as far as possible a rectangular profile with steep flanks (top hat), to achieve an optimum effect. However, the forming of a laser beam of this type is very complex. It has previously been realized either by use of special optics (for example diffractive optics) or with diaphragms in an imaging system in the path of rays. However, such optics can only be produced in a very complex way, while diaphragms cause a large part of the laser power, often over 50%, to be lost. The imaging with a fixed scale ratio that is required in this case also makes the path of rays very long, and requires very large deflecting mirrors in the deflecting unit.
An aim of an embodiment of the present invention is to provide a method of drilling holes, preferably using a perforated mask. In one embodiment, it is an aim to dispense with special optics and diaphragms, and achieve a uniform removal of material in the hole with little loss of laser power.
According to an embodiment of the invention, an aim is achieved by a method wherein the laser beam is moved in the region of the hole to be drilled on a circular path. The center point of the region lies concentrically with respect to the set position of the respective hole and the diameter of the region is smaller than or equal to the diameter of the hole. In one embodiment, the diameter of the laser beam spot is dimensioned such that it always covers the center point of the hole during the circular motion of the laser beam.
In a method according to an embodiment of the invention, the laser beam is consequently guided in what is known as a wobble motion on a circle within the hole, in such a way that at least the entire cross-sectional area of the hole is covered with each orbit. Further, the center point of the beam with the greatest energy density of the spot, follows an orbital path in the region between the center point or the set position of the center point and the edge. As such, the effective energy distribution is approximated to the desired rectangular shape.
On the basis of the energy distribution achieved according to an embodiment of the invention, it is also possible in principle to drill holes with adequate edge contours even without a perforated mask, in particular in organic or dielectric layers. A preferred embodiment of the invention may include the drilling of holes with the aid of perforated masks near the surface, with a metallization on the upper side of a (multilayer) printed circuit board preferably serving as the perforated mask, it likewise being possible for the holes in the perforated mask to be produced by laser drilling. In the case in which a perforated mask is used, a solution according to an embodiment of the invention is that the laser beam is moved in the region of the respective hole in the perforated mask on a circular path, the center point of which lies concentrically with respect to the set position of the respective hole in the mask and the diameter of which is smaller than or equal (at most equal to) to the diameter of the hole. Further, the diameter of the laser beam spot preferably always covers the center point of the hole in the mask during the circular motion of the laser beam, even in the case of a maximum permissible positional offset of the perforated mask.
To distribute the energy distribution as uniformly as possible over the circumference during the individual orbits of the laser beam, the laser beam is preferably guided in such a way that it always executes a whole number of complete circular orbits. The setting of the pulse energy of the laser is therefore chosen such that the total energy required for the drilling of the hole is distributed over a whole number of circular orbits of the laser beam.
A CO2 laser with high pulse frequencies, that is with repetition frequencies of 20 kHz to over 100 kHz, is preferably used for the method according to the invention, so that no time loss occurs in comparison with conventional methods. Small pulse widths of the order of magnitude of less than 500 ns, but typically xe2x89xa6150 ns, have the effect that the laser still has an adequate pulse power at these high repetition frequencies.